The thing about post-it notes is that they are really, really simple.
Even the simplest software is really, really complicated in comparison. Multiple orders of magnitude more complex. So the chance that you invent exactly the right piece of software, by accident, for a different market is highly unlikely.
What I see happen in practice is that a startup simply gives up on solving the original problem for the original customer because too many people say non-sensical things like ‘I think you’re targeting the wrong market’.
When you think about it, this is a really weird thing to say if you’re building a product to address a known customer need. It’s like saying ‘you’re losing at chess, how about golf’? Start with the need… and focus on that and avoid searching for the market that needs the product you’ve built.